I have also been bitten by this bug, but with an aerospire sensor. I wrote some new governor code for Copter 4.1, but when I went to test it, I get no rpm signal at all (testing on Copter 4.0.5). Reverting the firmware to Copter 3.6.12, the same sensor works fine.
I am using a Drotek Pixhawk3 Pro, not sure what controllers others are having problems with. But it doesn’t appear that the issue is limited to the Futaba sensor.